1. Field of the Invention
The present invention relates to a mixer for changing the frequency of a tuning signal in a television set which can receive both ground and satellite broadcasting signals.
2. Description of the Prior Art
A conventional mixer of the type referred to above has been constituted in such a structure such as that in FIG. 1. The mixer shown has a balun transformer T which has an input terminal A connected to a local oscillator, and a middle tap and one end of an unbalanced terminal grounded. The mixer also includes a terminal B which serves not only as an input terminal of a high frequency signal, but also as an output terminal of an intermediate frequency signal of both ground and satellite broadcasting signals, and diodes 13 and 14 for mixing frequencies. Nowadays, a ground broadcast signals can be received by a double superheterodyne tuner whereby the frequency of an input high frequency signal is changed to the 960 MHz band by a first frequency converting unit and then to a 58 MHz band by a second frequency converting unit. The mixer (first mixer) shown in FIG. 1 is used to change the input frequency to 960 MHz band.
The above-mentioned double superheterodyne tuner can receive ground broadcast and satellite broadcast signals, as well will be described with reference to FIG. 2.
In FIG. 2, a terminal E receives the ground broadcasting signal which is supplied to input bandpass filters 21, 22 and 23 which are respectively for VHF, CATV and UHF bands. The signal passing through one of these band-pass filters 21-23 is amplified by a wide-band amplifier 24 and input to a first mixer 25. The amplified signal is mixed in the first mixer 25 with a first local oscillation signal of a first local oscillator 26. Then, the difference component is output from the first mixer 25 as a first intermediate frequency signal (960 MHz band). The first intermediate frequency signal is amplified by a first IF amplifier 27. Only the first intermediate frequency signal passes through the bandpass filter 28 and is input to a second mixer 29, where the first intermediate frequency signal is mixed with a second local oscillation signal of a second local oscillator 30. The difference component is output from the second mixer 29 as a second intermediate frequency signal which passes through a second IF amplifier 31, and a second IF signal is produced from a terminal F.
A tuning voltage is supplied to the first local oscillator 26 from a terminal I. The first local oscillation signal of the first local oscillator 26 is frequency-divided by a pre-scaler 34, and the frequency-divided output is fed to a station selecting circuit (not shown) from a terminal J.
A terminal G is for the satellite broadcasting signal, and receives a signal (1.0-1.3 GHz) which has been changed to a first intermediate frequency at the preceding stage. Only the frequency of the satellite broadcast signal is allowed to pass through the input filter 32 to a switching circuit 37 via a first IF amplifier 33.
The switching circuit 37 changes its position so as to select and supply either the ground broadcasting or the satellite broadcasting signal to the first mixer 25. The second switching circuit 38 functions similarly to the first switching circuit 37, outputting the ground broadcasting signal to the first IF amplifier 27 and the satellite broadcasting signal to a second IF amplifier 35. An output signal from the first IF amplifier 33 is mixed with an input signal from the local oscillator 26 by the first mixer 25 which in turn generates the difference components (403 MHz) as a second IF signal. The second IF signal of 400 MHz is, after passing through the switching circuit 38 and the second IF amplifier 35, demodulated by an FM demodulator 36 and output to a terminal H. Therefore, the first mixer 25, first local oscillator 26 and pre-scaler 34 are used in common for both the ground and satellite broadcasting signals.
In the structure shown in FIGS. 1 and 2, the first local oscillation frequency when the ground broadcasting signal is received is 1050-1420 MHz, while the first intermediate frequency when the satellite broadcast is received is 1036-1331 MHz, resulting in overlapping of the band regions. In such an overlapping case, since the terminal B of FIG. 1 serves as a common input terminal for both the ground broadcasting and satellite broadcasting signals, there arises such a problem that, when the ground broadcasting signal is received by a first television set, the first intermediate frequency, i.e., 1036-1331 MHz of the satellite broadcasting signal received by a second television set having a common source line with the first television is disturbed by the local oscillation signal from the first television set. This can be understood such that, since the satellite broadcasting signal is in an FM system with little input power, the satellite broadcasting signal is susceptible to the leakage signal from the local oscillation signal. More specifically, for example, when a television set A of a first subscriber of the CATV system is tuned to a ground broadcasting channel and a television set B of a second subscriber of the CATV system is tuned to a satellite broadcasting channel, the local oscillation signal from the television set A is transmitted through the cable to the television set B resulting in the disturbance of the satellite broadcasting signal.